1) Field of the Invention
This invention relates generally to fabrication and design of metal fuses for semiconductor devices and particularly to the fabrication of a metal fuse having a guard ring composed of metal layers and vias.
2) Description of the Prior Art
Fuses can be used to rewire memory and logic circuits. For example, in dynamic or static memory chips, defective memory cells may be replaced by blowing fuses associated with the defective cells, and activating a spare row or column of cells. This circuit rewiring using fusible links allows considerable enhanced yields and reduces the production costs. Also, logic circuits may also be repaired or reconfigured by blowing fuses. For example, it is common to initially fabricate a generic logic chip having a large number of interconnected logic gates. Then, in a final processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry. Still other applications of laser-blown fuses are possible. There are two methods for blowing fuses: (a) using a laser and (b) passing a high current through the fuse. The portion of the fuse and thin insulating layer which is melted away or "blown" must not deposit or interfere with near-by devices. A laser is often used to break the fuse forming an electrical open by heating the fuse to a high temperature. It is conventional to have an opening through insulating over the fuse in the area where the fuse will be broken.
The inventor has found major problems in the process of forming opening (e.g., fuse openings 88) over fuses. As the number of metal layers and inter metal dielectric layers (IMD) increases, it is more difficult to accurately etch the fuse opening to a precise depth. Moreover, because of the additional inter metal dielectric layers, the dielectric layer uniformity within wafer and wafer to wafer creates difficulty in accurately making fuse openings.
Another important challenge is to improve the reliability and yields of the semiconductor devices surrounding openings in insulating layers, such as openings over fusible links. A problem with openings is that moisture and other contaminants can diffuse from the openings into the device areas thus reducing circuit reliability and yields.
The importance of overcoming the two major problems of (1) forming fuse openings of a uniform depth and (2) moisture diffusing through fuse windows to circuit areas, noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 4,536,949 (Takayama) that shows a method for a multi-layer wiring having opening for fuse. U.S. Pat. No. 5,567,643 (Lee) teaches a guard ring around a fuse opening. U.S. Pat. No. 5,457,059 (Keller) shows a metal fuse. U.S. Pat. No. 5,444,012 (Yoshizumi et al.) shows a guard ring. However, the first and second layers are not bonded to each other and therefore allow moisture to diffuse to the active devices. See Yoshizumi Figure 34. Unfortunately, the problem of contaminants diffusing to the semiconductor devices through the fuse window still exists and an improved structure/method of forming a guard ring is still needed.